What is the difference between an analogue and a fragment?

An analogue is a modified version of a parent; a fragment is an unmodified piece of a larger sequence. A molecule can be both.

What is a secretagogue?

A compound prompting a gland to release something it already produces, rather than supplying that substance directly.

Peptide Terminology Explained

The short answer

A small set of terms recurs throughout peptide literature, residue, terminus, amidation, analogue, fragment, agonist, secretagogue. Each has a precise meaning, and several are used loosely in commercial writing in ways that change what is being claimed.

Key facts

Residue: An amino acid within a chain
N-terminus: The free-amine end
C-terminus: The carboxyl end
Amidation: C-terminal COOH replaced by CONH2
Analogue: Modified version of a parent
Fragment: A piece of a larger sequence
Agonist: Activates a receptor

The counting prefixes

Peptides are frequently named by length in Greek numerals, tripeptide, pentapeptide, heptapeptide, nonapeptide, pentadecapeptide for fifteen. Seeing pentadecapeptide in a paper title tells you the molecule's size before you read anything else, which is why the convention persists.

Analogue versus fragment

These are different and are often conflated. An analogue is a modified version of a parent molecule. Semaglutide is a GLP-1 analogue, with substitutions and a lipid chain. A fragment is a piece of a larger sequence, unmodified but incomplete. TB-500 is a fragment of thymosin beta-4. A molecule can be both: Modified GRF (1-29) is a fragment of GHRH that is also an analogue of that fragment.

Research material referenced

BPC-157 5mg, third-party HPLC tested

Buy BPC-157 · £15.99

Why amidation is not cosmetic

Replacing a C-terminal carboxyl with an amide removes a negative charge and blocks carboxypeptidase attack. Many natural peptide hormones are amidated, so reproducing it in an analogue preserves both the native electrostatics and that protection. It also adds roughly 1 Da less than an unmodified terminus would give, which matters when checking a mass.

Agonist, antagonist and secretagogue

An agonist activates a receptor. An antagonist blocks it. A secretagogue prompts a gland to release something it already makes, which is a different kind of intervention from supplying the substance directly. The GIP paradox in the incretin literature (where an agonist and an antagonist both produce weight reduction) is a reminder that these labels describe mechanism rather than outcome.

Non-standard residues

Aib is α-aminoisobutyric acid, used to block peptidase access. D-amino acids are mirror images of the natural L forms and resist proteases that evolved for L backbones. Both appear in engineered peptides (ipamorelin contains one Aib and two D-residues) and neither can be written in single-letter code, which is itself a signal that a sequence is designed rather than natural.

Terms that carry a claim

Some names assert a function: body protection compound, delta sleep-inducing peptide, liver cell growth factor. These record what an investigator believed at discovery. They are hypotheses embedded in labels, and reading them as descriptions is the most common way a claim enters someone's thinking without ever being evaluated.

Frequently asked questions

What is the difference between an analogue and a fragment?: An analogue is a modified version of a parent; a fragment is an unmodified piece of a larger sequence. A molecule can be both.
Why are some peptides amidated?: It removes a C-terminal negative charge and blocks carboxypeptidase attack. Many natural peptide hormones are amidated.
What is a secretagogue?: A compound prompting a gland to release something it already produces, rather than supplying that substance directly.

Extended research context

The Peptide Reference deep dive

Deep dive: what 'peptide' actually means

A peptide is a short chain of amino acids linked by peptide bonds, typically 2–50 residues. Above that boundary the molecule is usually called a protein. Peptides can be endogenous (produced by the body) or synthetic (manufactured by solid-phase peptide synthesis, SPPS). The 'research peptide' category refers specifically to synthetic peptides supplied for laboratory use: not medicines, not supplements.

Why HPLC and mass spec together

HPLC (High-Performance Liquid Chromatography) reports the purity of a batch by measuring what percentage of the sample matches the target peptide's retention time. Mass spectrometry independently confirms the target's molecular weight. Together they answer two different questions: 'is it clean?' and 'is it the right molecule?'. A CoA that reports only one is incomplete.

How to read a Certificate of Analysis

A complete peptide CoA lists: batch number, HPLC purity (area %), mass-spec measured mass vs theoretical, water content (Karl Fischer), acetate/counterion content, appearance, and often endotoxin and residual solvents. Learn to spot the missing fields, as that's usually where quality claims fall apart.

Research applications

▸Reference standards for analytical method development
▸Comparator peptides in receptor-binding assays
▸Stability testing of lyophilised material
▸Formulation R&D for topical and aqueous carriers
▸Teaching material for peptide chemistry courses

Handling checklist

✓Confirm HPLC ≥98% and mass-spec identity on CoA
✓Store lyophilised at −20 °C long-term
✓Reconstitute with bacteriostatic or sterile water only
✓Aliquot to minimise freeze/thaw cycles
✓Label vials with date, concentration, and batch

Common research-handling mistakes

Learnt from thousands of researcher orders across our UK labs.

✗ Buying a peptide without a CoA

Fix: Insist on an in-batch HPLC + mass-spec certificate before purchase.

✗ Using DI water for reconstitution

Fix: Use bacteriostatic (0.9% benzyl alcohol) or sterile water only.

✗ Storing lyophilised vials at room temperature long-term

Fix: Freeze at −20 °C; short-term 2–8 °C is acceptable for weeks, not months.

Continue researching

Peer-reviewed guides, comparators and matched reference materials.

Primary sources & clinical trials

Peer-reviewed research and registered trials from PubMed, ClinicalTrials.gov, PubChem, FDA and NIH. All links open in a new tab and point to the primary source, so every claim can be verified at origin.

UKP

Written and reviewed by

The UK Peptides Editorial Team · Research library, UK Peptides

The editorial team is responsible for supplier selection, batch release decisions and the content published in this research library. Every article here is sourced to primary literature and every product page to a signed third-party certificate. Corrections are made in place and the review date updated.

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Peptide Terminology: The Words That Recur